Shanghai Ark Biopharmaceutical (ArkBio) has received approval from the US Food and Drug Administration (FDA) to begin a Phase II clinical trial of its antiviral drug ziresovir (AK0529) for use in severely ill infants infected with respiratory syncytial virus (RSV).

The randomised, multi-centre, placebo-controlled, double-blind study will focus on hospitalised babies in the paediatric intensive care unit (PICU), a group the company describes as having urgent treatment needs.

ArkBio has reported that ziresovir was the first direct-acting RSV antiviral to complete a Phase III clinical trial with positive results.

In the study, which involved children aged one to 24 months in China, ziresovir reduced viral load, the duration of respiratory symptoms, and the risk of recurrent wheezing and asthma over a 24-month follow-up period.

In 2025, the World Health Organization included ziresovir as the only therapeutic drug in its Paediatric Drug Optimization for Respiratory Syncytial Virus report.

The upcoming Phase II trial in the US will be a study designed to examine the drug’s clinical effectiveness in hospitalised infants, particularly measuring the duration of oxygen therapy and sustained recovery time.

ArkBio described the new trial as a milestone in its global development of ziresovir.

The company develops therapies for respiratory, lung and paediatric diseases. Its pipeline includes ziresovir; AK3280, an anti-fibrotic agent in Phase III trials for idiopathic pulmonary fibrosis; and the approved ADHD therapy AK0901 in China.

ArkBio collaborates with international partners such as Genentech, Qilu Pharmaceutical, Roche, and The Scripps Research Institute.

In October 2025, ArkBio commenced a randomised Phase II study to evaluate the fully human monoclonal antibody, AK0610, for the prevention of RSV infection.